1. Field of the Invention
This invention relates to a constant pressure printing mechanism for use in a portable label printing machine, or the like.
2. Description of the Prior Art
In conventional hand-held label printing machines, the hand operated lever is a driving lever. The printing lever, which carries a printing head, is generally integral with the driving lever. The driving lever is pivotally secured to the machine body. When the hand lever is squeezed, the printing head is moved down together with the printing lever until the type faces of the printing head are brought into contact with a label which is supported on a fixed platen.
The intensity of squeezing of the hand lever directly influences the printing pressure. Uneven squeezing force is liable to cause uneven darkness of the printed characters. Further, when the hand lever is squeezed strongly and quickly, the printing head strikes the platen hard and it bounces on the platen to cause double printing.
Previously, even when labels were printed somewhat indistinctly, they were generally accepted because they could be read with the naked eye when customers bought commodities or cashiers totalized sales accounts.
In recent years, however, what is called POS (point of sales) system has been employed throughout the world. In this system, the numerals, characters and symbols, such as bar codes and optically readable characters that are printed on labels, are automatically read out by means of optical character readers instead of the eyes of cashiers. These optical character readers memorize and calculate several management data such as stocks, sales, materials for customers in relation to commodities, and profit accounts. To be read by optical character readers, the labels must be printed clearly and precisely.